Princeton Filter Cap Replacement

[cyborg_stew]

I'm going to replace the original Mallory cap can on my '78, and I have a question before I order a new one.

There's a 22uF soldered from one of the leads of the can to ground. Is this wired in parallel in order to change the 1st filter cap to 42uF? Or is this added capacitor there to replace the original 20uF? I love how the amp sounds, so I want to keep the values the same as they are now.

[shooter]

appears to me

to change the 1st filter cap to 42uF

[sluckey]

More than likely the amp had started humming at one time. The added 22µF cap would have been the fix. It was common practice at one time to just bridge a cap across a dried out cap in the can rather than replace the entire can.

I love how the amp sounds, so I want to keep the values the same as they are now.

If that's the case I would suggest not doing anything to the amp. Wait until you don't love the sound so much. 

When the time come to replace the can there is an exact replacement available...

     https://www.tubesandmore.com/products/capacitor-ce-mfg-475v-20202020-f-electrolytic

[alerich]

I added a 22 uf cap across the first filter cap in my Princeton Reverb Reissues to beef things up a bit and make the bass notes less "farty". If I owned a older original I'd probably do the same since I really liked the effect. If you unsolder that extra cap and the amp starts to hum you'll know it was probably a cheap and quick repair. If not, maybe they liked the larger first filter, too.

[cyborg_stew]

Good idea alerich. I think I'll try desoldering the extra cap and see what happens.

I love how the amp sounds, but I don't love the hum that has started recently. lol. Did some basic diagnosing (with my limited knowledge), and I couldn't really find any glaring problems. I hadn't realized the cap can was original. I suspect that it's causing the hum. But if not, it's over 40 years old, so it should be replaced anyway.

Thanks guys!

[cyborg_stew]

I have one more question. The only cans I'm finding that are 20/20/20/20 or 40/20/20/20 are CE Manufacturing cans rated at 55C or 65C degrees. The original Mallory is rated for 85C. Would it be safe to use a 55C can?

[sluckey]

yes

[cyborg_stew]

What would happen if they get hotter than rated? Just shorten the lifespan?

[shooter]

What would happen if they get hotter than rated

you'll know you did something wrong in the build! 

I've built n fixed enough amps to be comfortable saying heat IS a result of excess current, NOT ambient operating temp, I do live in MI though 

[cyborg_stew]

I have a question for all of you knowledgeable people.

I installed a CE cap can in place of the original in my Princeton, because the amp had started humming recently. The hum is completely gone now. However, I was checking B+ voltages at the can, and 2 out of the 4 capacitors are reading significantly low. (They're numbers 3 and 4 in the circuit. The first 2 are fine.) The 4th filter cap supplies the plate voltages to V1. It should be getting 190VDC according to the schematic, and I only read 50VDC. 

I can't imagine that I did anything wrong in the installation, and I have no guesses as to what is causing this. Any help will be greatly appreciated.

[cyborg_stew]

Here's the schematic. It's hard to read some values, but the AA964 circuit is almost identical. https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Princeton-AA964-Schematic.pdf

[dude]

Perhaps the cap replacement had nothing to do with the lower voltage, maybe the voltages were low before the can change. I would check the rail resistors to start, the ones that are dropping the voltage. Usually these resistors drift high which would lower voltage.

[cyborg_stew]

I checked all resistors right before I took the old can out, and I've measured voltage throughout the circuit recently. With the problem being so early in the circuit, I wouldn't think anything further down the line could affect it.

[dude]

I wouldn't think anything further down the line could affect it.

If your voltages are good till the tone stack and preamp first stage. Then the problem lies there. Did you check the plate resistors? Match the voltages you have from where they go low against schematic with correct vintages and at what points are there differences?

[PRR]

> The 4th filter cap supplies the plate voltages to V1. It should be getting 190VDC according to the schematic, and I only read 50VDC.

The CAP should sit at 290V. The plate at 190V.

So I don't know what is going on.

[shooter]

and I only read 50VDC

what happens with NO tubes, what's the vdc?
add 1 tube ?
2 tube ?
........

[HotBluePlates]

... I installed a CE cap can in place of the original in my Princeton ... and 2 out of the 4 capacitors are reading significantly low. (They're numbers 3 and 4 in the circuit. The first 2 are fine.) The 4th filter cap supplies the plate voltages to V1. It should be getting 190VDC according to the schematic, and I only read 50VDC. ...

You changed nothing else when swapping the cap?  No changed power supply resistors?

Because if you had swapped out the 18kΩ resistors and accidentally installed higher values, that would explain the low voltage.  A short-circuit somewhere (in the preamp) drawing too much current from the 4th filter cap would also cause low voltage readings.

[cyborg_stew]

I wouldn't think anything further down the line could affect it.

If your voltages are good till the tone stack and preamp first stage. Then the problem lies there. Did you check the plate resistors? Match the voltages you have from where they go low against schematic with correct vintages and at what points are there differences?

The voltage goes awry much earlier than that in the circuit. At the 3rd filter cap, the it's much less than it should be.

They plate resistors for v1 check out. Both are at 102k.

[cyborg_stew]

The only other thing I did was swap out two ceramic disc .01 capacitors in the tremolo circuit, as one of had become microphonic. When tapping on it with a chopstick, sound came thru the speaker.

I didn't change the power supply resistors. That's the first thing I suspected though. When checking them with a meter, they all measured well within tolerance.

Is it possible that I inadvertently messed something up when changing out the tremolo caps? Maybe causing a short, like HotBluePlates mentioned?

[dude]

If the two rail R's are 18K and plates 100k then maybe a short is draining current
like HBP mentioned. Use continuity with your meter, ground to the 3rd and fourth stage area for a short. Possible a tube went, did you pull tubes like shooter mentioned and check voltages with first, then second 12ax out? With out tubes what are voltages on the 100k plates.
Maybe when you changed the trem cap, you cause a short, maybe solder blob under the board touching chassis...

[cyborg_stew]

Thank you all for your help! It's greatly appreciated, and nice to know there are people who are willing to share their knowledge with others.

After spending alot of time trying to diagnose where the hum was coming from, then replacing the cap can and fixing that problem, only to have another occur; I decided to take the amp to my local tech. I'm in over my head, and I thought it best to leave it to the pros.

Thanks again!

[dude]

Let us know what it was when your tech fixes it, usually it's the last thing you did...